Diabetes Mellitus: Types, Diagnosis and Complications

Diabetes Mellitus: Types, Diagnosis and Complications

Diabetes mellitus is a condition in which the body either does not produce enough insulin or cannot respond to insulin normally, causing blood glucose levels to remain abnormally high. Between 11% and 14% of adults worldwide have diabetes, and the condition is a leading cause of cardiovascular disease, kidney failure, blindness and lower-limb amputation when left unmanaged.

How Blood Sugar and Insulin Work

The carbohydrates in food are broken down in the digestive tract into simple sugars, principally glucose. Glucose is absorbed into the bloodstream and serves as the body's primary fuel source.

Insulin — a hormone produced by the pancreas — enables glucose to move from the blood into cells. When blood glucose rises after a meal, the pancreas releases more insulin. As glucose returns to baseline, insulin output falls. Blood glucose changes with meals and other factors. Fasting and post-meal measurements have different reference ranges; a fasting-like range should not be applied throughout the day.

When the pancreas cannot produce sufficient insulin, or when cells stop responding to it normally (insulin resistance), glucose accumulates in the blood rather than reaching the cells that need it. This is diabetes.

Types of Diabetes

Type 1 Diabetes

In type 1 diabetes, the immune system attacks and destroys the insulin-producing beta cells in the pancreas. The result is little or no insulin production. Type 1 diabetes can develop at any age, including later adulthood; age alone should not be used to rule it out. People with type 1 diabetes require lifelong insulin therapy.

The exact trigger is not fully understood, but a combination of genetic susceptibility and early-life environmental factors — such as viral infections or nutritional exposures — is thought to initiate the autoimmune process.

Type 2 Diabetes

Type 2 diabetes accounts for 90–95% of all diabetes cases. The pancreas continues to produce insulin — sometimes at above-normal levels early in the disease — but the body's cells become resistant to its effects. Over time, the pancreas loses capacity and insulin output declines.

Type 2 diabetes was once considered a condition of middle age, but it is increasingly diagnosed in younger adults and adolescents. Risk rises progressively after age 30 and continues to increase with age.

Certain medications and medical conditions can impair insulin utilisation and contribute to type 2 diabetes, including:

• High-dose corticosteroids (e.g. prednisone, prednisolone)
• Cushing's syndrome
• Some antipsychotic drugs and protease inhibitors

Prediabetes

Prediabetes refers to blood glucose levels that are above the normal range but below the threshold for a diabetes diagnosis. People with prediabetes carry a higher risk of developing type 2 diabetes as well as cardiovascular disease. A 5–10% reduction in body weight through diet and exercise significantly reduces the likelihood of progression.

Diagnostic thresholds for prediabetes:

• Fasting blood glucose: 100–125 mg/dL (5.6–6.9 mmol/L)
• 2-hour glucose after oral glucose tolerance test (OGTT): 140–199 mg/dL (7.8–11.0 mmol/L)
• HbA1c: 5.7%–6.4%

Gestational Diabetes

Gestational diabetes develops during pregnancy because of the insulin resistance caused by pregnancy-related hormonal changes. It generally resolves after delivery, but women who have had gestational diabetes face a markedly higher lifetime risk of developing type 2 diabetes.

Less Common Types

Other forms of diabetes include:

• Monogenic diabetes – caused by single-gene defects affecting insulin secretion or action
• Latent autoimmune diabetes in adults (LADA) – an autoimmune form with slower progression than typical type 1, sometimes initially misdiagnosed as type 2
• Pancreatogenic diabetes (type 3c) – resulting from pancreatitis, haemochromatosis, or surgical removal of the pancreas
• Drug-induced diabetes – associated with glucocorticoids, beta-blockers, calcineurin inhibitors, immune checkpoint inhibitors, and certain antiretrovirals

Symptoms

Diabetes symptoms vary by type and can be subtle, particularly in type 2 diabetes:

• Increased thirst (polydipsia) – elevated blood glucose draws fluid from tissues, causing dehydration
• Frequent urination (polyuria) – the kidneys excrete excess glucose through urine
• Unexplained weight loss – more pronounced in type 1; the body breaks down fat and muscle when glucose cannot enter cells
• Fatigue – cells are deprived of their primary energy source
• Blurred vision – osmotic changes alter the shape of the eye lens
• Slow-healing wounds and recurrent infections – high glucose impairs immune function and circulation
• Tingling or numbness in hands and feet – an early sign of nerve damage

Many people with type 2 diabetes have no symptoms for years. The condition is frequently discovered incidentally during routine blood tests.

Diagnosis

Fasting Plasma Glucose (FPG)

Blood is drawn after an overnight fast of at least 8 hours.

Result	Interpretation
Below 100 mg/dL (5.6 mmol/L)	Normal
100–125 mg/dL (5.6–6.9 mmol/L)	Prediabetes
126 mg/dL (7.0 mmol/L) or above	Diabetes

Random Blood Glucose

A random (non-fasting) blood glucose of 200 mg/dL (11.1 mmol/L) or above, in the presence of symptoms, is sufficient for a diabetes diagnosis.

HbA1c (Glycated Haemoglobin)

HbA1c reflects the average blood glucose level over the preceding 2–3 months. Unlike fasting glucose tests, it does not require preparation and can be performed at any time of day. It must be measured in a certified laboratory.

HbA1c Level	Interpretation
Below 5.7%	Normal
5.7%–6.4%	Prediabetes
6.5% or above	Diabetes

In the absence of unequivocal hyperglycaemia, diagnosis generally requires confirmatory abnormal testing. Classic hyperglycaemic symptoms with a random plasma glucose of at least 200 mg/dL (11.1 mmol/L), or a hyperglycaemic crisis, can establish the diagnosis without waiting for repeat confirmation. The clinician interprets the result in context.

Oral Glucose Tolerance Test (OGTT)

Outside pregnancy, a common OGTT protocol measures fasting blood glucose and glucose 2 hours after a standard glucose drink to help diagnose diabetes or impaired glucose tolerance. Testing during pregnancy follows specific screening and diagnostic protocols, with different sampling schedules and interpretation. Follow the maternity team’s instructions; this general description is not a complete gestational-diabetes testing protocol.

Complications of Uncontrolled Diabetes

Sustained elevated blood glucose causes progressive damage to blood vessels and nerves throughout the body. Complications are broadly categorised as acute or long-term.

Acute Complications

• Hypoglycaemia – dangerously low blood glucose, usually from excess insulin or medication
• Diabetic ketoacidosis (DKA) – occurs primarily in type 1 diabetes when the body breaks down fat for energy, producing acidic ketone bodies
• Hyperosmolar hyperglycaemic state (HHS) – a serious emergency in type 2 diabetes, characterised by very high blood glucose without significant ketosis

Long-Term Complications

Organ or System	Complication	Consequence
Eyes	Diabetic retinopathy	Vision loss and blindness
Kidneys	Diabetic nephropathy	Chronic kidney disease and kidney failure
Nerves	Diabetic neuropathy	Numbness, pain, and loss of sensation, particularly in the feet
Heart	Coronary artery disease	Heart attack and heart failure
Brain	Cerebrovascular disease	Stroke
Legs and feet	Peripheral artery disease	Impaired circulation, ulcers, amputation

High blood glucose also suppresses immune function, making people with diabetes significantly more susceptible to bacterial and fungal infections.

Treatment Principles

Type 1 Diabetes

Treatment requires lifelong insulin replacement. Regimens combine rapid-acting insulin at mealtimes with a long-acting basal insulin to mimic normal pancreatic function. Continuous glucose monitors (CGMs) and insulin pumps allow more precise glucose control and are increasingly standard of care.

Type 2 Diabetes

Management follows a stepwise approach:

1. Lifestyle intervention — diet modification, regular physical activity, and weight loss form the foundation of treatment. A 5–10% reduction in body weight significantly improves insulin sensitivity.
2. Non-insulin medicines — treatment is individualised. Options include oral drugs such as metformin, DPP-4 inhibitors and SGLT-2 inhibitors, and GLP-1 receptor agonists, many of which are injections; an oral form of semaglutide also exists. Cardiovascular disease, heart failure, kidney disease, weight, side effects and access affect selection. Benefits differ by medicine and indication.
3. Insulin therapy — may be needed when other treatment is insufficient, but also at presentation in some people with marked symptomatic hyperglycaemia, weight loss or a hyperglycaemic crisis. It should not be delayed merely to try every oral medicine first.

Considerations for Older Adults

Strict blood glucose targets that are appropriate for younger patients carry a higher risk of hypoglycaemia in frail or elderly individuals. Falls, confusion, and loss of consciousness from hypoglycaemia can be particularly dangerous in this group. Treatment goals are therefore individualised based on overall health status, life expectancy and other medical conditions.

Diabetes Screening in China

China's major tertiary hospitals — including nationally recognised institutions in Beijing, Shanghai, Guangzhou and Xi'an — operate specialist endocrinology and metabolic disease departments capable of delivering:

• A clinician-selected diabetes assessment, with the necessary tests, preparation and result timing confirmed by the hospital
• Multidisciplinary evaluation involving endocrinology, cardiology, nephrology and ophthalmology where indicated
• Itemised hospital and PriumCare charges confirmed separately before you commit; confirm the payment route for each charge under the current service terms

If you have a family history of diabetes, are experiencing symptoms, or are seeking a thorough health assessment before they become a problem, contact a PriumCare care navigator to arrange an appointment with full English-language coordination.

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Background source: MSD Manual Professional: Overview of Diabetes Mellitus, by Erika F. Brutsaert, MD, peer reviewed by Glenn D. Braunstein, MD. Its full review is dated December 2025, with an update in April 2026. These names and dates credit the external source, not authorship or medical review of this PriumCare article.

Additional editorial source clarification

Editorial clarification: 25 October 2026. This targeted update addresses the specific diagnostic, treatment-selection or service claims identified in the sources below. It is not a clinical review of the whole article; the original publication date is retained.

• Source 1
• Source 2

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Targeted editorial source check — 26 October 2026: Only the type 1 diabetes age-of-onset wording was checked against NIH/NIDDK information and NICE adult diagnosis guidance. This is not a whole-article medical review, adds no named clinical reviewer and retains the original publication date.

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Targeted editorial source check — 26 October 2026: The OGTT paragraph distinguishes general testing from pregnancy-specific protocols, based on NIH/NIDDK diabetes testing information and gestational-diabetes testing information. This is not a whole-article medical review or an individual testing instruction.